The requirements for the signal quality of modulators, for example in transmitting devices, become more stringent as the need for high data rates and increasing mobility grows. The modern mobile radio standards, such as universal mobile telecommunication system (UMTS), wideband code division multiple access (WCDMA), global system for mobile communication (GSM), enhanced data rates for GSM evolution (EDGE), Bluetooth medium data rate or wireless local area network (WLAN) according to 802.11a/b/g require special modulation types for data transmission which modulate both the phase and the amplitude of a carrier signal at the same time.
Simultaneous amplitude and phase modulation make it possible to achieve higher data transmission rates and thus better bandwidth efficiency. The mobile radio standards mentioned above envisage, for example, the use of quadrature phase shift keying (QPSK), eight phase shift keying (8-PSK), or quadrature amplitude modulation (QAM) as modulation types for the data transmission.
Depending on the selected application for the individual mobile radio standards, these high-quality modulation types are used not only for data transmission from a base station to a mobile communication appliance but also from the mobile communication appliance to the base station.
The modulation types which are used for modern mobile radio standards are particularly sensitive to possible interference or distortion which is produced by various components in a transmission path. The modulation can be performed using a vector modulation in which data to be transmitted are provided with an in-phase component and a quadrature component which form a complex signal. In other systems, also a polar modulator can be used in which the data are coded as polar coordinates with an amplitude component and a phase component. The amplitude component is digital/analog converted and used for an amplitude modulation of a carrier signal which comprises the phase information of the phase component. Although the binary data of the phase component usually comprise an equal distribution of ones and zeros, it is possible that a permanent DC offset is generated, for example, in a digital/analog conversion step. When modulating the carrier signal with the analog amplitude signal having a DC offset the signal quality of the modulated signal can be impaired.
It is possible to determine the offset generated between the digital/analog converter and the amplitude modulator in a transmission path and to generate a signal to compensate for the offset. To this end, a compensation signal is adapted over several steps until the offset in the analog signal is compensated. Because of transient effects of the amplitude signal between the digital/analog converter and the amplitude modulator, a constant time between adjustment steps is chosen sufficiently long. This can lead to problems for time critical applications because of the duration of the approximation process.